In a typical system, a memory controller bridges communications between a system bus and a processor bus, and essentially acts as a master response agent (i.e. the target) of processor initiated transactions to read data from I/O devices coupled to the system bus. This works well when all I/O agents are on the system bus, i.e. they are located downstream of the memory controller.
However, some systems exist wherein a I/O agent may be located north or upstream of a memory controller. An example of such a system is a system that integrates an I/O device such as a communications processor into a central processing unit (CPU). With these systems the memory controller is unaware of I/O devices north of it and may therefore incorrectly claim cycles for transactions intended for those I/O devices.
This problem can be overcome by controlling the behavior of the memory controller in software. However, not all operating systems allow this type of control.